The application of conversion coating solutions to render a surface black is a common technique being widely applied to zinc and zinc alloy layers including zinc-cobalt, zinc-nickel and zinc-iron layers. Zinc and zinc alloy layers may be applied by hot dip galvanizing but are most commonly applied by electroplating from plating solutions.
Conversion coatings applied to a zinc or zinc alloy layer rendering a surface black are common to the field and comprise a basic chromium(III) complex and an oxidation agent in an acidic solution.
These formulations, also referred to as passivates form a chromium(III) based passivation layer with black pigment particles generated in situ. The chromium(III)-complex based layers increase corrosion protection already provided by the zinc or zinc alloy layer and the black pigments in the passivation layer render the surface of the coated substrate black. The additional corrosion protection provided by the chromium(III)-passivate layer is caused by a barrier function delaying the access of any corrosive solution to the zinc or zinc alloy layer.
Unfortunately, black pigmented passivate layers do not bear the same corrosion protection like it is found in non pigmented, so called clear or iridescent passivate layers. The black pigments do not contribute to corrosion protection and to some extend may interfere with the barrier functionality.
This results in a more permeable structure of the black passivate layer in turn leading to earlier formation of undesired white corrosion on the surface (white rust). Those white corrosion products on the surface form a thin, dense layer improving the barrier function of the passivate layer and thereby resulting in a self inhibition of the corrosion which usually stops on the level of a thin, haze like white cover with corrosion products. The optical appearance of such a black surface is not sufficient anymore after formation of white rust.
This effect can particularly be observed on the surface of black passivated zinc-nickel alloy layers which usually have a nickel concentration of 12 to 15 wt.-%. The nickel concentration range is chosen in order to obtain the best cathodic corrosion resistance to substrates made of iron-containing materials at a sufficiently slow corrosion rate to reach 720 h to iron corrosion (formation of red rust) at 8 μm thickness of the zinc-nickel alloy layer as determined in the neutral salt spray test according to ISO 9227 NSS. However, white rust formed already at an early stage alters the optical appearance of the black surface in an undesired manner by formation of e.g. white haze.
A higher nickel concentration in the zinc-nickel alloy layer inevitably leads to premature red corrosion due to localized galvanic corrosion with no or very low cathodic protection potential. Typically, such substrates covered with a zinc-nickel alloy layer of >16 wt.-% nickel undergo very early punctual red corrosion rendering such a high nickel concentration in a zinc-nickel alloy layer useless.